In some installations it is desired to pasteurize batches of various different products such as different types of ice cream mix and the like, and where the batch size varies over a wide range. In vat type pasteurization apparatus, the product to be pasteurized is heated in a vat to a temperature in the range of 63 to 69 degrees C. and held at that temperature for about thirty minutes. The vat pasteurization method is relatively slow because of the time required for heating the batch up to pasteurization temperature and the relatively long holding time. In addition, vat pasteurization apparatus is not suitable for relatively large batches since it is necessary to either provide a very large vat that can accommodate the entire batch, or to pasteurize the batch in smaller quantities with a consequent increase in the overall pasteurization time.
Continuous-flow type pasteurization apparatus are commonly used in large milk processing plants. In such continuous-flow type pasteurization apparatus, the product to be pasteurized is pumped by a positive displacement pump through a heating heat exchanger where the product is rapidly heated to a pasteurization temperature, for example in the range of 72 to 80 degrees C. and maintained at that temperature for a short time of the order of fifteen to thirty seconds, and the pasteurized product is then passed through a cooling heat exchanger where it is cooled. Such continuous-flow type pasteurization apparatus work well for pasteurizing very large quantities of product where the pasteurization apparatus can work continuously for long periods. However, the continuous-flow type pasteurization apparatus presents some problems when used for pasteurizing small or moderate batches of product, and particularly relatively viscous products such as ice cream mix and the like, because of the amount of product that remains in the pasteurization apparatus at the end of each batch and which quantity is lost or wasted when the pasteurizer is subsequently cleaned.